Microwave and millimeter-wave reflectarrays are antenna arrays in which each antenna is connected to a variable impedance device. If the variable impedance device is a resistive device, ideally it is biased at either short-circuit or open-circuit states or at least at states of very low or very high resistance compared to the antenna impedance. In this way, the antenna and variable impedance device combination acts as a variable-phase reflecting element. With appropriate control signals applied to the variable impedance devices, the reflectarray can be used as an adjustable mirror with the capability of focusing, steering, or splitting a beam of electromagnetic radiation.
Diodes have typically been used as the variable impedance devices in reflectarrays although transistors have also been used. Many low cost transistors are able to provide the desired impedance characteristics at relatively low radio frequency ranges (e.g., less than 10 GHz). However, these low cost transistors do not necessarily provide the desired impedance performance characteristics at relatively high radio frequency ranges (e.g., greater than 20 GHz). More expensively packaged transistors as well as more exotic devices, such as ferroelectric capacitors and ferrite switches, are readily able to provide the desired impedance characteristics at the higher frequency ranges, but the number of transistors, capacitors, or switches required for many reflectarray applications combined with the high cost of each packaged device makes reflectarrays with such devices prohibitively expensive.
In view of this, what is needed is a reflectarray that exhibits the desired impedance characteristics and that is economical to implement.